1. Field of the Invention
The present invention relates to an image processing method and an image processing system wherein a combination of images including various types of color or halftone images, such as graphic images, continuous tone images, text and so on, is converted into a binary bitmap image by using halftoning techniques.
2. Description of the Related Art
When a combination of images including text, graphic images and continuous tone images is printed by a printer, the combination of images is converted into a binary bitmap image. If the combination of images includes halftone images or color images, halftoning techniques are used for the conversions of them.
Text and graphic images are generated by personal computers or word processors. The text and the graphic images are written in page description language. When the text and the graphic images are printed by the printer, they are converted into multi-level tone value images, each pixel of which has a multi-level tone value, such as a value that ranges between 0 and 255 (i.e., 8 bits value). Further, the multi-level tone value images are converted into the binary bitmap images by using a simple halftoning technique, such as an ordered dither method, a dot pattern method, or the like.
On the other hand, continuous tone images, such as photographs, are scanned by an image scanner and stored into a memory device of a personal computer. When the continuous tone images are printed by the printer, they are converted into binary bitmap images by using a halftoning technique that is different from the halftoning technique used for the conversion of text and graphic images. In case of continuous tone images, an error diffusion method or a minimum average error method is used for the purpose of increasing the quality of reproduction of continuous tone images.
Therefore, when a combination of images including color or halftone graphic images, continuous tone images and text is printed by the printer, it is converted into a binary bitmap image by using at least two kinds of halftone technique, depending on whether each image is graphic image, continuous tone image or text.
A dither pattern is used in the dithering process. The properties of the dither pattern affect tone and resolution of a binary bitmap image generated in the dithering process. If the user can selects a suitable dither pattern from among various dither patterns, it is possible to increase the quality of the binary bitmap image.
However, if the dither pattern is changed, the text and the graphic images have to be converted again into the multi-level tone value images, and the multi-level tone value images have to be further converted again into the binary bitmap images. And also, the continuous tone images have to be converted again into the binary bitmap images. Thus, each time the dither pattern is changed, the converting process wherein text and graphic images are converted into multi-level tone value images, the converting process wherein the multi-level tone value images are converted into binary bitmap images, and the converting process wherein continuous tone images are converted into binary bitmap images have to be performed again, regardless of that the original text, graphic images and continuous tone images are not changed. As a result, each time the dither pattern is changed, it takes a long time for the converting processes.